This invention relates to a numerical control system for controlling a machine tool, and in particular an improved numerical control system capable of starting a control from a corrected present position of the machine tool irrespective of a power failure.
A numerical control system, as well known, is adapted to control a machine tool to permit the latter to machine a workpiece into a desired shape. A positioning control and continuous cutting control are known in such a numerical control system.
A numerical control is effected by reading by means of a tape reader an instruction tape in which an instruction relating to a movement path of a machine tool is punched, storing the instruction in an input memory, comparing an instruction data stored in the input memory with a data corresponding to the present position of the machine tool, and driving the machine tool by a drive mechanism into an instruction position based on the result of comparison.
An open loop control system (using a pulse motor) and a closed loop control system are known as a drive system for a machine tool. The closed system is adapted to feedback a data corresponding to the present position of the machine tool using a position detector. The open loop control system is adapted to updata the contents of a present position memory by the number of pulses delivered to a drive mechanism, while on the other hand the closed loop control system is adapted to update the contents of a present position memory according to a signal corresponding to an actual position of a position detector which is mounted on the machine tool. In the latter system, therefore, a high precision system results.
In the above mentioned conventional numerical control system an integral circuit constructed of semiconductor logical elements is used as a memory for storing a data corresponding to the present position of the machine tool. When a machining operation is interrupted or a power supply is interrupted due to a power failure, the present position data stored in the memory is destroyed. When the machining operation is to be resumed, it is necessary to move a movable section of the machine tool to an original point. Therefore, the conventional numerical control needs to have the function for returning the movable section of the machine tool to the original position so as to obtain an accurate position of the machine tool when a power supply to the numerical control system is resumed. This makes the readjustment of the tool position more difficult.